The global Earthquakes in the event of an impact with something the size of Earth would wrench and liquefy the planet all the way around to the other side of the world in waves of destruction which would occur for many many years as the objects stabilized into a new planet. A giant ripple of Earths mantle would sweep around the world as the planets collided and utterly destroy the crust layers. The point opposite the impact would be the convergence point for the ripples running around the world over and over again.. the earth would get thoroughly stirred up in such an event.

the elastic collision of two giant bodies would minimize the g forces felt as both objects would distort heavily and merge in the process. You would most likely die as a result of the crust layers overturning and getting stirred into the molten rock below.

The atmosphere would be incinerated and reconfigured as the oceans and icecaps flash into steam and gigatons of gasses like sulfer dioxode and carbon monoxide are stirred out of the mantle to bubble to the surface.

Edit: If we define gargantuan i'll revise my statements to match. To me gargantuan is planet sized, while massive would be like a continent, big something like a mountain (a few thosand meters), small would be a few hundred meters

If a big enough object hits with enough force you could possibly get gigantic eruptions on the opposite side of the planet. There's a bit of debate in the scientific community whether or not this is actually possible.

These huge eruptions form what are called traps which are caused by giant magma plumes rising to the surface. Some of these traps that have been discovered were formed due to eruptions lasting hundreds of thousands of years.

There are debates around this as to whether the KT event (the asteroid that wiped out the dinosaurs), wiped out the dinosaurs alone or if it to formed a trap on the other side of the planet which would have contributed to the extinction. Traps have been found which formed at the same time of massive extinctions. The problem is linking them to asteroid impacts which occured at the same time and seems to be a bit controversial.